IN-VITRO AND IN-VIVO STUDIES ON TRANSTHYRETIN OLIGOMERIZATION

The oligomerization of transthyretin has been studied in vitro and in vivo. The results showed that wild-type transthyretin synthesized in v itro in the absence of microsomes did not form dimers and tetramers, b ut in the presence of microsomes the mature transthyretin which had be en translocated into the microsomal lumen formed dimers and a small am ount of tetramers which could be detected only by using a cross-linkin g reagent. Efficiency of tetramer formation depends upon the source of microsomes; in fact the amount of tetramer formed in liver microsomes was much higher than that in pancreas microsomes. Transthyretin synth esized in HepG2 cells appeared after SDS-PAGE analysis in mostly tetra meric form, while that synthesized in transfected COS-1 cells appeared mainly as dimers. Brefeldin A treatment and pulse-chase experiments i n HepG2 cells showed that transthyretin tetramer was formed in the end oplasmic reticulum. These results strongly indicate that transthyretin tetramer is formed most efficiently in the endoplasmic reticulum lume n of hepatocytes. Transthyretin without the signal peptide [S(-)] and transthyretin with a mutation that prevents processing of the signal p eptide Mse failed to form dimers and tetramers in vitro. In the transf ected COS-l cells, however, S(-) transthyretin did form dimers while M se transthyretin failed to oligomerize. These results show that the cl eavage of the signal peptide and some cellular factors are required fo r transthyretin oligomerization. (C) 1998 Academic Press.